Air filters frequently use granular filter materials to filter air passing through the filter and thereby remove unwanted substances from the air. By way of example, many air filters use activated carbon granules to purge unwanted substances (e.g., toxins, biomolecules, etc.) from the air. Typically, the granular filter materials are disposed in a frame, between two screens (e.g., meshes, grills, etc.) which pass air but retain the granular filter materials.
The granular filter materials typically have an irregular shape which can complicate close packing. More particularly, during shipping and handling of the filter, the granular filter materials tend to shift and settle, which can result in the creation of preferential passageways through the filter materials.
Furthermore, as the granular filter materials shift within the filter, larger granules can break down into smaller granules, which can in turn lead to further shifting and settling, thereby leading to additional granule breakdown, etc. This problem is particularly acute where the granular filter materials are relatively brittle and hence susceptible to breakage. As the granular filter materials break down into smaller and smaller particles, the tendency to create preferential passageways increases.
The aforementioned preferential passageways through the granular filter materials (which are sometimes referred to as “chimneys”) are generally undesirable, since they permit the air to pass through the filter with reduced exposure to the filter materials, thereby reducing the effectiveness of the filter. This shifting and settling effect is particularly troublesome in the case of activated carbon granules and the like, since these granules tend to be irregularly shaped and hence highly susceptible to the aforementioned detrimental shifting and settling during shipping and handling.
As a result, it is common for granular filter manufacturers, and in particular activated carbon filter manufacturers, to use various means to reduce detrimental shifting and settling of the granular filter materials within the filter, e.g., shifting and settling during shipping and handling. Such filter material stabilizing means are well known in the industry, and include (i) the use of compression springs and elastic foam to compress the granular filter materials between the two opposing screens, whereby to reduce detrimental shifting and settling of the granular filter materials; (ii) cross-bracing the frame (i.e., across the depth of the filter) so as to subdivide the granular filter materials into smaller, independent domains, whereby to reduce shifting and settling of the granular filter materials during shipping and handling; (iii) limiting the size (i.e., the cross-sectional area) of each filter, and using more individual filters, to provide a given level of filtration (i.e., to provide a given cross-sectional filtration area); etc.
While these approaches have been found to generally reduce the formation of preferential passageways within the granular filter materials and thereby increase the effectiveness of the filter, such constructions are generally more complex to manufacture and hence increase the overall cost of the filter. In addition, while the use of cross-bracing can help to reduce shifting and settling of the granular filter materials by subdividing the granular filter materials into smaller, independent domains, it also tends to increase the number of filter granules which are positioned alongside a wall of the cross-bracing. This presents a new problem, since the irregularly-shaped granules tend to pack poorly alongside the walls of the cross-bracing, thereby leading to the creation of “wall chimneys” (i.e., chimneys running the depth of the filter, alongside the walls of the cross-bracing). Inasmuch as the cross-bracing walls extend through the full depth of the filter, these “wall chimneys” create substantial new preferential passageways extending through the depth of the filter and thus reduce the overall effectiveness of the filter.
Accordingly, there is a need for a new and improved filter construction which provides means for reducing detrimental shifting and settling of the granular filter materials during shipping and handling, while minimizing the complexity, cost and wall chimney problems encountered by the prior art.